A cocktail of chemicals in the air is making saplings grow bigger than their country cousins.

Identical trees planted 100 kilometres out of the Big Apple only reached half the size of city dwellers in a study.

Ecologists in the United States say the airborne chemical "footprint" seems to favour trees growing in cities.

"City-grown pollution, and ozone in particular, is tougher on country trees," says Cornell University ecologist Jillian Gregg.

Chemical scavengers

Ground-level ozone is a secondary pollutant created by the action of sunlight on primary pollutants created in cities.

City-grown pollution is tougher on country trees

Dr Jillian Gregg, Cornell University

Ozone levels can build up rapidly in conurbations. But other chemicals can act as "scavengers", reacting with the ozone and causing levels to drop to almost zero at night and in the winter.

But when ozone is blown into rural areas, cumulative exposure can be greater than in the city centre.

This is because the scavengers, such as nitric oxide churned out by cars, are less abundant outside the urban setting. These higher ozone levels have a detrimental effect on rural trees, leading to stunted growth.

"This [study] looked at one species in one city," Dr Gregg told BBC News Online. "But the species is mid-range in terms of ozone sensitivity. We expect that all ozone species should show the same response."

Town vs country

The research looked at a native North American tree, the Eastern cottonwood or poplar (Populus deltoides), which grows mainly in mid-western states on flood plains.

Once established, it shoots up rapidly, reaching 30 metres (100 feet) or more in 35 years.

Pollution has a pay-off for trees

Dr Gregg and colleagues took identical root cuttings of the tree and grew them in urban and rural sites in and around New York City.

They wanted to see how the strains of city life affected plants. A big surprise was in store, however.

After three seasons, the urban cottonwoods had thrived, weighing twice as much as rural ones.

The researchers were able to rule out the influence on growth rates of factors other than ozone such as soil composition, temperature, and carbon dioxide.

Writing in the journal Nature, they say the results do not negate the known detrimental effects of urban pollutants.

Rather, they highlight the effects of secondary reactions that can create high ozone levels beyond the urban core.